As part of the on-going evolution of the third Generation Partnership Project (3GPP) Wideband Code Division Multiple Access (WCDMA) standard, Dual-Cell High-Speed Downlink Packet Access (HSDPA) (DC-HSDPA) has been approved in 3GPP. Dual-cell HSDPA is a natural evolution of High Speed Packet Access (HSPA) which allows the use of a second HSPA carrier (i.e. two 5 MHz downlink carriers) to create a bigger downlink data pipe.
The DC-HSDPA operation is backward compatible with Release 7, 6, and 5, and Release 99 devices through seamless interoperation between single-carrier and dual-carrier coverage areas. Dual-cell operation provides both throughput increase and latency reduction. Most importantly, more wireless transmit receive units (WTRUs) have access to higher data rates, especially in poor radio conditions where techniques such as Multiple Input Multiple Output (MIMO) are not used. In terms of system performance, Dual-Cell HSDPA offers efficient load balancing across carriers and some capacity gain.
The agreed dual-cell operation in Release 8 of the 3GPP standard only applies to the downlink, with the uplink (UL) transmission restricted to a single cell, i.e., carrier. Furthermore, the following additional restrictions have been imposed: the two downlink cells belong to the same Node-B and are on adjacent carriers (and by extension the carriers are in the same frequency band); two carriers operating in the dual-cell have the same time reference and their downlinks are synchronized; and the two downlink cells cover the same geographical area (sector). Accordingly, a dual-cell capable WTRU is configured to receive two downlink carriers (an anchor carrier and a supplementary carrier), and to transmit one uplink anchor carrier. The downlink anchor carrier is matched with the uplink anchor carrier.
Additionally, inter-frequency handovers can be used to change anchor carriers within a Node-B.
A Dual Cell HSDPA WTRU may be configured to perform normal mobility procedures. An important aspect of HSDPA and enhanced dedicated channel (E-DCH) mobility is the serving cell change (handover). Handover is the process in which a WTRU switches from one cell to another without service interruption. Soft handover refers to a feature where a WTRU is simultaneously connected to two or more cells (or cell sectors) during a call. If the sectors are from the same physical cell site (a sectorized site), it is referred to as softer handover.
In HSDPA, the handover procedure does not allow for soft handover or softer handover. The high-speed shared channels are monitored by the WTRU in a single cell, which is called the serving HS-DSCH cell. During handover, the WTRU switches to a new serving HS-DSCH cell (target cell/Node B) and stops communication with the old serving HS-DSCH cell (source cell/Node B). This procedure is also called serving HS-DSCH cell change.
With the introduction of the enhanced DCH in the UL, the WTRU must also maintain a connection with a serving E-DCH cell. The serving HS-DSCH cell and serving E-DCH cell must be identical throughout the WTRU connection. Therefore, when a serving HS-DSCH cell change occurs, a serving E-DCH cell change also occurs. The combined procedure is also referred to as the serving cell change.
An important aspect in handover is the selection of a “best cell”. Accordingly, the WTRU continuously measures the signal strength of the common pilot channel (CPICH) of the neighboring cells. If the measured signal of the neighboring cell exceeds that of the serving cell, the WTRU reports to the radio network controller (RNC) a change of best cell via a Radio Resource Controller (RRC) measurement report event 1D. The measurement report contains the measured value and cell identification (cell ID). The RNC then makes the final determination as to whether a serving cell change should occur.
A serving cell change can also occur via other RRC measurement report events, such as event 1A or event 1C, or as part of an active set update procedure.
Upon reception of these events, the RNC determines whether to perform a handover to a new cell. The serving RNC (SRNC) requests the controlling RNC (CRNC) to allocate high-speed downlink shared channel (HS-DSCH) resources (e.g. as HS-DSCH radio network transaction identifier (H-RNTI), high-speed shared control channel (HS-SCCH) codes, hybrid automatic repeat-request (HARQ) resources, etc.) and E-DCH resources (such as, E-RNTI, E-DCH Absolute Grant Channel (E-AGCH) and serving E-DCH Relative Grant Channel (E-RGCH), etc.) for the WTRU in the target cell via Radio Network Subsystem Application Part (RNSAP) and/or Node-B Application Part (NBAP) messages. Once the resources are reserved, the CRNC provides all the information to the SRNC which in turn transmits an RRC handover message to the WTRU. The RRC message, which can indicate a serving HS-DSCH cell change includes, but is not limited, to: a physical channel reconfiguration, transport channel reconfiguration, radio bearer reconfiguration, and active set update.
The RRC handover message provides the WTRU with the radio access parameters required for the WTRU to start monitoring the target cell. In addition, the RRC message may provide an activation time which notifies the WTRU at which time the handover should occur.
Handovers can be synchronized or unsynchronized. In an unsynchronized handover the network and the WTRU do not activate the resources and switch at the same time. The activation time for the WTRU is set to “now”. This reduces the delays associated with the handover procedure; however it increases the probability of losing data.
In a synchronized handover, the network and the WTRU perform the change of resources simultaneously. The network sets the activation time to a conservative value to account for any kind of delays such as scheduling delay, retransmissions, configuration time etc. While the synchronized handovers minimize data losses, it does result in higher delays.
The RRC handover message is transmitted to the WTRU via the source Node-B. The delay associated with the serving HS-DSCH cell change procedure may cause the handover message to fail, thus resulting in an unacceptable rate of dropped calls. As a result, to optimize the serving HS-DSCH cell procedure, a pre-loading (pre-configuration) of the WTRU and the Node-B with HS-DSCH or E-DCH related configuration has been proposed. When a cell is added to the active set, the WTRU and the Node-B are pre-configured with the radio link (RL) reconfiguration prepare/ready phase. When a change in the best cell occurs (i.e. an event 1D), the configuration of the target Node-B, which is already pre-configured, can be activated by the RNC.
Parallel monitoring of the source Node-B HS-SCCH and the target Node-B HS-SCCH has also been proposed. Upon a change of the best cell, the WTRU transmits an event 1D measurement report. After waiting for a configurable amount of time, the WTRU starts monitoring the pre-loaded target Node-B's HS-SCCH in addition to the HS-SCCH of the source Node-B. In performing these steps the service discontinuity is reduced.
Another alternative to optimize the serving HS-DSCH cell procedure is for the WTRU only to monitor one cell at a time. Once an event 1D is triggered, the WTRU provides the network with the time at which the handover will occur, i.e. the connection frame number (CFN), in the measurement report message. At the given CFN, the WTRU will then stop monitoring the source cell and move to the target cell.
Implicit re-pointing to the target Node-B at a first scheduling occurrence may also be used. When the RNC authorizes the handover and the target Node-B is configured and ready, the RNC can schedule the WTRU on one of the HS-SCCHs that is monitored by the WTRU. The first scheduling occurrence from the target Node-B implicitly confirms a successful handover, thus a handover complete message is transmitted to the RNC. To avoid packet loss, the source Node-B can provide the RNC a status message indicating the amount of data that still needs to be transmitted.
The handover (or re-pointing) indication can also be transmitted over the target Node-B, via an HS-SCCH order, or a serving cell change channel (SCCCH), which uses the same channelization code as the E-RGCH and E-DCH HARQ Acknowledgement Indicator Channel (E-HICH) but with a different signature sequence.
The WTRU acknowledges the handover indication by changing the UL scrambling code, or by using a special value of the channel quality indicator (CQI) (i.e. 31) or the Scheduling Information (SI).
The introduction of a second carrier in the downlink impacts existing mobility procedures. The enhancements to the serving cell change procedure have been optimized in the context of single carrier operation. When a second carrier is introduced the enhanced serving cell change procedure does not take into account both carriers. Therefore, there exists a need for an improved method and apparatus for dual serving cell change.